Fix scope in `publish` error handler

Fix crash:

    npm ERR! TypeError: Cannot call method 'error' of undefined
    npm ERR!     at handle (/Users/maciej/dev/js/npm/node_modules/npm-registry-client/lib/publish.js:79:18)
    npm ERR!     at RegClient.<anonymous> (/Users/maciej/dev/js/npm/node_modules/npm-registry-client/lib/publish.js:88:24)
    npm ERR!     at cb (/Users/maciej/dev/js/npm/node_modules/npm-registry-client/lib/request.js:27:9)
    npm ERR!     at RegClient.<anonymous> (/Users/maciej/dev/js/npm/node_modules/npm-registry-client/lib/request.js:148:10)
    npm ERR!     at cb (/Users/maciej/dev/js/npm/node_modules/npm-registry-client/lib/request.js:158:9)
    npm ERR!     at RegClient.<anonymous> (/Users/maciej/dev/js/npm/node_modules/npm-registry-client/lib/request.js:223:20)
    npm ERR!     at self.callback (/Users/maciej/dev/js/npm/node_modules/request/main.js:120:22)
    npm ERR!     at Request.EventEmitter.emit (events.js:124:20)
    npm ERR!     at ClientRequest.self.clientErrorHandler (/Users/maciej/dev/js/npm/node_modules/request/main.js:222:10)
    npm ERR!     at ClientRequest.EventEmitter.emit (events.js:94:17)
v0.2.18